Subcutaneous cardiac signal discrimination employing non-electrophysiologic signal

ABSTRACT

A cardiac monitoring and/or stimulation system includes a housing coupled to a plurality of electrodes configured for subcutaneous non-intrathoracic sensing. A signal processor receives a plurality of composite signals associated with a plurality of sources, separates a signal from the plurality of composite signals, and identifies the separated signal as a cardiac signal using information derived from a non-electrophysiologic sensor, such as an accelerometer or acoustic transducer. The signal processor may iteratively correlate separated signals from the plurality of composite signals with a non-electrophysiologic sensor signal until the cardiac signal is identified.

RELATED APPLICATIONS

This application claims the benefit of Provisional Patent ApplicationSer. No. 60/462,272, filed on Apr. 11, 2003, to which priority isclaimed pursuant to 35 U.S.C. §119(e) and which is hereby incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to subcutaneous cardiac sensing and/orstimulation devices employing cardiac signal separation.

BACKGROUND OF THE INVENTION

The healthy heart produces regular, synchronized contractions. Rhythmiccontractions of the heart are normally controlled by the sinoatrial (SA)node, which is a group of specialized cells located in the upper rightatrium. The SA node is the normal pacemaker of the heart, typicallyinitiating 60-100 heartbeats per minute. When the SA node is pacing theheart normally, the heart is said to be in normal sinus rhythm.

If the heart's electrical activity becomes uncoordinated or irregular,the heart is denoted to be arrhythmic. Cardiac arrhythmia impairscardiac efficiency and can be a potential life-threatening event.Cardiac arrhythmias have a number of etiological sources, includingtissue damage due to myocardial infarction, infection, or degradation ofthe heart's ability to generate or synchronize the electrical impulsesthat coordinate contractions.

Bradycardia occurs when the heart rhythm is too slow. This condition maybe caused, for example, by impaired function of the SA node, denotedsick sinus syndrome, or by delayed propagation or blockage of theelectrical impulse between the atria and ventricles. Bradycardiaproduces a heart rate that is too slow to maintain adequate circulation.

When the heart rate is too rapid, the condition is denoted tachycardia.Tachycardia may have its origin in either the atria or the ventricles.Tachycardias occurring in the atria of the heart, for example, includeatrial fibrillation and atrial flutter. Both conditions arecharacterized by rapid contractions of the atria. Besides beinghemodynamically inefficient, the rapid contractions of the atria mayalso adversely affect the ventricular rate.

Ventricular tachycardia occurs, for example, when electrical activityarises in the ventricular myocardium at a rate more rapid than thenormal sinus rhythm. Ventricular tachycardia can quickly degenerate intoventricular fibrillation. Ventricular fibrillation is a conditiondenoted by extremely rapid, uncoordinated electrical activity within theventricular tissue. The rapid and erratic excitation of the ventriculartissue prevents synchronized contractions and impairs the heart'sability to effectively pump blood to the body, which is a fatalcondition unless the heart is returned to sinus rhythm within a fewminutes.

Implantable cardiac rhythm management systems have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically include one or more leads and circuitry to sense signals fromone or more interior and/or exterior surfaces of the heart. Such systemsalso include circuitry for generating electrical pulses that are appliedto cardiac tissue at one or more interior and/or exterior surfaces ofthe heart. For example, leads extending into the patient's heart areconnected to electrodes that contact the myocardium for sensing theheart's electrical signals and for delivering pulses to the heart inaccordance with various therapies for treating arrhythmias.

Typical Implantable cardioverter/defibrillators (ICDs) include one ormore endocardial leads to which at least one defibrillation electrode isconnected. Such ICDs are capable of delivering high-energy shocks to theheart, interrupting the ventricular tachyarrhythmia or ventricularfibrillation, and allowing the heart to resume normal sinus rhythm. ICDsmay also include pacing functionality.

Although ICDs are very effective at preventing Sudden Cardiac Death(SCD), most people at risk of SCD are not provided with implantabledefibrillators. Primary reasons for this unfortunate reality include thelimited number of physicians qualified to perform transvenouslead/electrode implantation, a limited number of surgical facilitiesadequately equipped to accommodate such cardiac procedures, and alimited number of the at-risk patient population that may safely undergothe required endocardial or epicardial lead/electrode implant procedure.

SUMMARY OF THE INVENTION

The present invention is directed to cardiac monitoring and/orstimulation methods and systems that, in general, provide transthoracicmonitoring, defibrillation therapies, pacing therapies, or a combinationof these capabilities. Embodiments of the present invention are directedto subcutaneous cardiac monitoring and/or stimulation methods andsystems that detect and/or treat cardiac activity or arrhythmias.

According to one embodiment of the invention, a medical system includesa housing having a medical device disposed within the housing. Thehousing is coupled to a plurality of electrodes configured forsubcutaneous non-intrathoracic sensing. A signal processor is coupled tothe plurality of electrodes and configured to receive a plurality ofcomposite signals associated with a plurality of sources sensed by atleast some of the electrodes. The signal processor is further configuredto separate a signal from the plurality of composite signals andidentify the separated signal as a cardiac signal.

In another embodiment of the present invention, a system includes ahousing coupled to a plurality of electrodes configured for subcutaneousnon-intrathoracic sensing. A signal processor receives a plurality ofcomposite signals associated with a plurality of sources, separates asignal from the plurality of composite signals using blind sourceseparation, and identifies a cardiac signal using information from anon-electrophysiological cardiac source, such as from an accelerometer,acoustic transducer or other sensor that senses cardiac activity otherthan cardiac electrophysiology signals. The signal processor mayiteratively correlate separated signals from the plurality of compositesignals with the non-electrophysiological cardiac source signal untilthe cardiac signal is identified.

An embodiment of a method of signal separation involves detecting aplurality of composite signals at a plurality of locations, separating asignal using blind source separation, and identifying a cardiac signalusing both electrical and non-electrical cardiac information. The methodmay also involve providing a detection window defined by a start timeand a stop time determined using the non-electrophysiological cardiacsource information. The QRS complex may be detected within the detectionwindow.

The method may involve using acoustic emission information such as atemporal location of a peak heart-sound. The detection window may bedefined by a start time preceding the temporal location of a peakheart-sound. The method may involve the use of othernon-electrophysiological cardiac source information such as blood-flowinformation, pulse pressure information, and/or pulse oximetryinformation such as the pulse oximetry information generated usingphotoplethysmography. The method may also involve identifying theseparated signal as the cardiac signal by providing a detection windowwithin which the cardiac signal is correlated to thenon-electrophysiological cardiac source.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views of a transthoracic cardiac sensing and/orstimulation device as implanted in a patient in accordance with anembodiment of the present invention;

FIG. 1C is a block diagram illustrating various components of atransthoracic cardiac sensing and/or stimulation device in accordancewith an embodiment of the present invention;

FIG. 1D is a block diagram illustrating various processing and detectioncomponents of a transthoracic cardiac sensing and/or stimulation devicein accordance with an embodiment of the present invention;

FIG. 2 is a diagram illustrating components of a transthoracic cardiacsensing and/or stimulation device including an electrode array inaccordance with an embodiment of the present invention;

FIG. 3 is a block diagram illustrating uses of signal separation inaccordance with the present invention;

FIG. 4 is a block diagram of a cardiac sensing methodology incorporatingsignal separation in accordance with the present invention;

FIG. 5 is a block diagram of a signal separation process in accordancewith the present invention;

FIG. 6 is a graph illustrating the results of a signal separationprocess in accordance with the present invention;

FIG. 7 is a pictorial diagram of a carotid pulse waveform, aphonocardiogram (PCG) waveform, an electrocardiogram (ECG) waveform, anda filtered transthoracic impedance signal for two consecutiveheartbeats;

FIG. 8 is a graph illustrating two consecutive pseudo PQRS complexes andtheir associated pseudo accelerometer signals, and a detection windowfor correlation of the signals in accordance with an embodiment of thepresent invention; and

FIG. 9 is a flow chart of a method of signal separation in accordancewith the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings, which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

An implanted device according to the present invention may include oneor more of the features, structures, methods, or combinations thereofdescribed hereinbelow. For example, a cardiac monitor or a cardiacstimulator may be implemented to include one or more of the advantageousfeatures and/or processes described below. It is intended that such amonitor, stimulator, or other implanted or partially implanted deviceneed not include all of the features described herein, but may beimplemented to include selected features that provide for uniquestructures and/or functionality. Such a device may be implemented toprovide a variety of therapeutic or diagnostic functions.

In general terms, a cardiac signal detection arrangement and method maybe used with a subcutaneous cardiac monitoring and/or stimulationdevice. One such device is an implantable transthoracic cardiac sensingand/or stimulation (ITCS) device that may be implanted under the skin inthe chest region of a patient. The ITCS device may, for example, beimplanted subcutaneously such that all or selected elements of thedevice are positioned on the patient's front, back, side, or other bodylocations suitable for sensing cardiac activity and delivering cardiacstimulation therapy. It is understood that elements of the ITCS devicemay be located at several different body locations, such as in thechest, abdominal, or subclavian region with electrode elementsrespectively positioned at different regions near, around, in, or on theheart.

The primary housing (e.g., the active or non-active can) of the ITCSdevice, for example, may be configured for positioning outside of therib cage at an intercostal or subcostal location, within the abdomen, orin the upper chest region (e.g., subclavian location, such as above thethird rib). In one implementation, one or more electrodes may be locatedon the primary housing and/or at other locations about, but not indirect contact with the heart, great vessel or coronary vasculature.

In another implementation, one or more leads incorporating electrodesmay be located in direct contact with the heart, great vessel orcoronary vasculature, such as via one or more leads implanted by use ofconventional transvenous delivery approaches. In a furtherimplementation, for example, one or more subcutaneous electrodesubsystems or electrode arrays may be used to sense cardiac activity anddeliver cardiac stimulation energy in an ITCS device configurationemploying an active can or a configuration employing a non-active can.Electrodes may be situated at anterior and/or posterior locationsrelative to the heart. Examples of useful subcutaneous electrodes,electrode arrays, and orientations of same are described in commonlyowned U.S. patent application Ser. No. 10/738,608 entitled “NoiseCanceling Cardiac Electrodes,” filed Dec. 17, 2003, and U.S. patentapplication Ser. No. 10/465,520 filed Jun. 19, 2003 entitled “MethodsAnd Systems Involving Subcutaneous Electrode Positioning Relative To AHeart”, which are hereby incorporated herein by reference.

Certain configurations illustrated herein are generally described ascapable of implementing various functions traditionally performed by animplantable cardioverter/defibrillator (ICD), and may operate innumerous cardioversion/defibrillation modes as are known in the art.Exemplary ICD circuitry, structures and functionality, aspects of whichmay be incorporated in an ITCS device of a type that may benefit fromsignal separation in accordance with the present invention, aredisclosed in commonly owned U.S. Pat. Nos. 5,133,353; 5,179,945;5,314,459; 5,318,597; 5,620,466; and 5,662,688, which are herebyincorporated herein by reference in their respective entireties.

In particular configurations, systems and methods may perform functionstraditionally performed by pacemakers, such as providing various pacingtherapies as are known in the art, in addition tocardioversion/defibrillation therapies. Exemplary pacemaker circuitry,structures and functionality, aspects of which may be incorporated in anITCS device of a type that may benefit from signal separation, aredisclosed in commonly owned U.S. Pat. Nos. 4,562,841; 5,284,136;5,376,106; 5,036,849; 5,540,727; 5,836,987; 6,044,298; and 6,055,454,which are hereby incorporated herein by reference in their respectiveentireties. It is understood that ITCS device configurations may providefor non-physiologic pacing support in addition to, or to the exclusionof, bradycardia and/or anti-tachycardia pacing therapies.

An ITCS device in accordance with the present invention may implementdiagnostic and/or monitoring functions as well as provide cardiacstimulation therapy. Exemplary cardiac monitoring circuitry, structuresand functionality, aspects of which may be incorporated in an ITCSdevice of a type that may benefit from signal separation in accordancewith the present invention, are disclosed in commonly owned U.S. Pat.Nos. 5,313,953; 5,388,578; and 5,411,031, which are hereby incorporatedherein by reference in their respective entireties.

An ITCS device may be used to implement various diagnostic functions,which may involve performing rate-based, pattern and rate-based, and/ormorphological tachyarrhythmia discrimination analyses. Subcutaneous,cutaneous, and/or external sensors may be employed to acquirephysiologic and non-physiologic information for purposes of enhancingtachyarrhythmia detection and termination. It is understood thatconfigurations, features, and combination of features described in thepresent disclosure may be implemented in a wide range of implantablemedical devices, and that such embodiments and features are not limitedto the particular devices described herein.

Referring now to FIGS. 1A and 1B of the drawings, there is shown aconfiguration of a transthoracic cardiac sensing and/or stimulation(ITCS) device having components implanted in the chest region of apatient at different locations. In the particular configuration shown inFIGS. 1A and 1B, the ITCS device includes a housing 102 within whichvarious cardiac sensing, detection, processing, and energy deliverycircuitry may be housed. It is understood that the components andfunctionality depicted in the figures and described herein may beimplemented in hardware, software, or a combination of hardware andsoftware. It is further understood that the components and functionalitydepicted as separate or discrete blocks/elements in the figures may beimplemented in combination with other components and functionality, andthat the depiction of such components and functionality in individual orintegral form is for purposes of clarity of explanation, and not oflimitation.

Communications circuitry is disposed within the housing 102 forfacilitating communication between the ITCS device and an externalcommunication device, such as a portable or bed-side communicationstation, patient-carried/worn communication station, or externalprogrammer, for example. The communications circuitry may alsofacilitate unidirectional or bidirectional communication with one ormore external, cutaneous, or subcutaneous physiologic or non-physiologicsensors. The housing 102 is typically configured to include one or moreelectrodes (e.g., can electrode and/or indifferent electrode). Althoughthe housing 102 is typically configured as an active can, it isappreciated that a non-active can configuration may be implemented, inwhich case at least two electrodes spaced apart from the housing 102 areemployed.

In the configuration shown in FIGS. 1A and 1B, a subcutaneous electrode104 may be positioned under the skin in the chest region and situateddistal from the housing 102. The subcutaneous and, if applicable,housing electrode(s) may be positioned about the heart at variouslocations and orientations, such as at various anterior and/or posteriorlocations relative to the heart. The subcutaneous electrode 104 iscoupled to circuitry within the housing 102 via a lead assembly 106. Oneor more conductors (e.g., coils or cables) are provided within the leadassembly 106 and electrically couple the subcutaneous electrode 104 withcircuitry in the housing 102. One or more sense, sense/pace ordefibrillation electrodes may be situated on the elongated structure ofthe electrode support, the housing 102, and/or the distal electrodeassembly (shown as subcutaneous electrode 104 in the configuration shownin FIGS. 1A and 1B).

In one configuration, the lead assembly 106 is generally flexible andhas a construction similar to conventional implantable, medicalelectrical leads (e.g., defibrillation leads or combineddefibrillation/pacing leads). In another configuration, the leadassembly 106 is constructed to be somewhat flexible, yet has an elastic,spring, or mechanical memory that retains a desired configuration afterbeing shaped or manipulated by a clinician. For example, the leadassembly 106 may incorporate a gooseneck or braid system that may bedistorted under manual force to take on a desired shape. In this manner,the lead assembly 106 may be shape-fit to accommodate the uniqueanatomical configuration of a given patient, and generally retains acustomized shape after implantation. Shaping of the lead assembly 106according to this configuration may occur prior to, and during, ITCSdevice implantation.

In accordance with a further configuration, the lead assembly 106includes a rigid electrode support assembly, such as a rigid elongatedstructure that positionally stabilizes the subcutaneous electrode 104with respect to the housing 102. In this configuration, the rigidity ofthe elongated structure maintains a desired spacing between thesubcutaneous electrode 104 and the housing 102, and a desiredorientation of the subcutaneous electrode 104/housing 102 relative tothe patient's heart. The elongated structure may be formed from astructural plastic, composite or metallic material, and includes, or iscovered by, a biocompatible material. Appropriate electrical isolationbetween the housing 102 and subcutaneous electrode 104 is provided incases where the elongated structure is formed from an electricallyconductive material, such as metal.

In one configuration, the rigid electrode support assembly and thehousing 102 define a unitary structure (e.g., a single housing/unit).The electronic components and electrode conductors/connectors aredisposed within or on the unitary ITCS device housing/electrode supportassembly. At least two electrodes are supported on the unitary structurenear opposing ends of the housing/electrode support assembly. Theunitary structure may have an arcuate or angled shape, for example.

According to another configuration, the rigid electrode support assemblydefines a physically separable unit relative to the housing 102. Therigid electrode support assembly includes mechanical and electricalcouplings that facilitate mating engagement with correspondingmechanical and electrical couplings of the housing 102. For example, aheader block arrangement may be configured to include both electricaland mechanical couplings that provide for mechanical and electricalconnections between the rigid electrode support assembly and housing102. The header block arrangement may be provided on the housing 102 orthe rigid electrode support assembly. Alternatively, amechanical/electrical coupler may be used to establish mechanical andelectrical connections between the rigid electrode support assembly andhousing 102. In such a configuration, a variety of different electrodesupport assemblies of varying shapes, sizes, and electrodeconfigurations may be made available for physically and electricallyconnecting to a standard ITCS device housing 102.

It is noted that the electrodes and the lead assembly 106 may beconfigured to assume a variety of shapes. For example, the lead assembly106 may have a wedge, chevron, flattened oval, or a ribbon shape, andthe subcutaneous electrode 104 may include a number of spacedelectrodes, such as an array or band of electrodes. Moreover, two ormore subcutaneous electrodes 104 may be mounted to multiple electrodesupport assemblies 106 to achieve a desired spaced relationship amongstsubcutaneous electrodes 104.

An ITCS device may incorporate circuitry, structures and functionalityof the subcutaneous implantable medical devices disclosed in commonlyowned U.S. Pat. Nos. 5,203,348; 5,230,337; 5,360,442; 5,366,496;5,397,342; 5,391,200; 5,545,202; 5,603,732; and 5,916,243, which arehereby incorporated herein by reference in their respective entireties.

FIG. 1C is a block diagram depicting various components of an ITCSdevice in accordance with one configuration. According to thisconfiguration, the ITCS device incorporates a processor-based controlsystem 205 which includes a micro-processor 206 coupled to appropriatememory (volatile and non-volatile) 209, it being understood that anylogic-based control architecture may be used. The control system 205 iscoupled to circuitry and components to sense, detect, and analyzeelectrical signals produced by the heart and deliver electricalstimulation energy to the heart under predetermined conditions to treatcardiac arrhythmias. In certain configurations, the control system 205and associated components also provide pacing therapy to the heart. Theelectrical energy delivered by the ITCS device may be in the form of lowenergy pacing pulses or high-energy pulses for cardioversion ordefibrillation.

Cardiac signals are sensed using the subcutaneous electrode(s) 214 andthe can or indifferent electrode 207 provided on the ITCS devicehousing. Cardiac signals may also be sensed using only the subcutaneouselectrodes 214, such as in a non-active can configuration. As such,unipolar, bipolar, or combined unipolar/bipolar electrode configurationsas well as multi-element electrodes and combinations of noise cancelingand standard electrodes may be employed. The sensed cardiac signals arereceived by sensing circuitry 204, which includes sense amplificationcircuitry and may also include filtering circuitry and ananalog-to-digital (A/D) converter. The sensed cardiac signals processedby the sensing circuitry 204 may be received by noise reductioncircuitry 203, which may further reduce noise before signals are sent tothe detection circuitry 202.

Noise reduction circuitry 203 may also be incorporated after sensingcircuitry 202 in cases where high power or computationally intensivenoise reduction algorithms are required. The noise reduction circuitry203, by way of amplifiers used to perform operations with the electrodesignals, may also perform the function of the sensing circuitry 204.Combining the functions of sensing circuitry 204 and noise reductioncircuitry 203 may be useful to minimize the necessary componentry andlower the power requirements of the system.

In the illustrative configuration shown in FIG. 1C, the detectioncircuitry 202 is coupled to, or otherwise incorporates, noise reductioncircuitry 203. The noise reduction circuitry 203 operates to improve thesignal-to-noise ratio (SNR) of sensed cardiac signals by removing noisecontent of the sensed cardiac signals introduced from various sources.Typical types of transthoracic cardiac signal noise includes electricalnoise and noise produced from skeletal muscles, for example.

Detection circuitry 202 typically includes a signal processor thatcoordinates analysis of the sensed cardiac signals and/or other sensorinputs to detect cardiac arrhythmias, such as, in particular,tachyarrhythmia. Rate based and/or morphological discriminationalgorithms may be implemented by the signal processor of the detectioncircuitry 202 to detect and verify the presence and severity of anarrhythmic episode. Exemplary arrhythmia detection and discriminationcircuitry, structures, and techniques, aspects of which may beimplemented by an ITCS device of a type that may benefit from signalseparation in accordance with the present invention, are disclosed incommonly owned U.S. Pat. Nos. 5,301,677 and 6,438,410, which are herebyincorporated herein by reference in their respective entireties.

The detection circuitry 202 communicates cardiac signal information tothe control system 205. Memory circuitry 209 of the control system 205contains parameters for operating in various sensing, defibrillation,and, if applicable, pacing modes, and stores data indicative of cardiacsignals received by the detection circuitry 202. The memory circuitry209 may also be configured to store historical ECG and therapy data,which may be used for various purposes and transmitted to an externalreceiving device as needed or desired.

In certain configurations, the ITCS device may include diagnosticscircuitry 210. The diagnostics circuitry 210 typically receives inputsignals from the detection circuitry 202 and the sensing circuitry 204.The diagnostics circuitry 210 provides diagnostics data to the controlsystem 205, it being understood that the control system 205 mayincorporate all or part of the diagnostics circuitry 210 or itsfunctionality. The control system 205 may store and use informationprovided by the diagnostics circuitry 210 for a variety of diagnosticspurposes. This diagnostic information may be stored, for example,subsequent to a triggering event or at predetermined intervals, and mayinclude system diagnostics, such as power source status, therapydelivery history, and/or patient diagnostics. The diagnostic informationmay take the form of electrical signals or other sensor data acquiredimmediately prior to therapy delivery.

According to a configuration that provides cardioversion anddefibrillation therapies, the control system 205 processes cardiacsignal data received from the detection circuitry 202 and initiatesappropriate tachyarrhythmia therapies to terminate cardiac arrhythmicepisodes and return the heart to normal sinus rhythm. The control system205 is coupled to shock therapy circuitry 216. The shock therapycircuitry 216 is coupled to the subcutaneous electrode(s) 214 and thecan or indifferent electrode 207 of the ITCS device housing. Uponcommand, the shock therapy circuitry 216 delivers cardioversion anddefibrillation stimulation energy to the heart in accordance with aselected cardioversion or defibrillation therapy. In a lesssophisticated configuration, the shock therapy circuitry 216 iscontrolled to deliver defibrillation therapies, in contrast to aconfiguration that provides for delivery of both cardioversion anddefibrillation therapies. Exemplary ICD high energy delivery circuitry,structures and functionality, aspects of which may be incorporated in anITCS device of a type that may benefit from aspects of the presentinvention are disclosed in commonly owned U.S. Pat. Nos. 5,372,606;5,411,525; 5,468,254; and 5,634,938, which are hereby incorporatedherein by reference in their respective entireties.

In accordance with another configuration, an ITCS device may incorporatea cardiac pacing capability in addition to cardioversion and/ordefibrillation capabilities. As is shown in dotted lines in FIG. 1C, theITCS device may include pacing therapy circuitry 230, which is coupledto the control system 205 and the subcutaneous and can/indifferentelectrodes 214, 207. Upon command, the pacing therapy circuitry deliverspacing pulses to the heart in accordance with a selected pacing therapy.Control signals, developed in accordance with a pacing regimen bypacemaker circuitry within the control system 205, are initiated andtransmitted to the pacing therapy circuitry 230 where pacing pulses aregenerated. A pacing regimen may be modified by the control system 205.

A number of cardiac pacing therapies may be useful in a transthoraciccardiac monitoring and/or stimulation device. Such cardiac pacingtherapies may be delivered via the pacing therapy circuitry 230 as shownin FIG. 1C. Alternatively, cardiac pacing therapies may be delivered viathe shock therapy circuitry 216, which effectively obviates the need forseparate pacemaker circuitry.

The ITCS device shown in FIG. 1C is configured to receive signals fromone or more physiologic and/or non-physiologic sensors in accordancewith embodiments of the present invention. Depending on the type ofsensor employed, signals generated by the sensors may be communicated totransducer circuitry coupled directly to the detection circuitry 202 orindirectly via the sensing circuitry 204. It is noted that certainsensors may transmit sense data to the control system 205 withoutprocessing by the detection circuitry 202.

Non-electrophysiological cardiac sensors may be coupled directly to thedetection circuitry 202 or indirectly via the sensing circuitry 204.Non-electrophysiological cardiac sensors sense cardiac activity that isnon-electrophysiological in nature. Examples of non-electrophysiologicalcardiac sensors are blood oxygen sensors, blood volume sensors, acousticsensors and/or pressure transducers, and accelerometers. Signals fromthese sensors are developed based on cardiac activity, but are notderived directly from electrophysiological sources (e.g., R-waves orP-waves). A non-electrophysiological cardiac sensor 261, as isillustrated in FIG. 1C, may be connected to one or more of the sensingcircuitry 204, detection circuitry 202 (connection not shown forclarity), and the control system 205.

Communications circuitry 218 is coupled to the microprocessor 206 of thecontrol system 205. The communications circuitry 218 allows the ITCSdevice to communicate with one or more receiving devices or systemssituated external to the ITCS device. By way of example, the ITCS devicemay communicate with a patient-worn, portable or bedside communicationsystem via the communications circuitry 218. In one configuration, oneor more physiologic or non-physiologic sensors (subcutaneous, cutaneous,or external of patient) may be equipped with a short-range wirelesscommunication interface, such as an interface conforming to a knowncommunications standard, such as Bluetooth or IEEE 802 standards. Dataacquired by such sensors may be communicated to the ITCS device via thecommunications circuitry 218. It is noted that physiologic ornon-physiologic sensors equipped with wireless transmitters ortransceivers may communicate with a receiving system external of thepatient.

The communications circuitry 218 may allow the ITCS device tocommunicate with an external programmer. In one configuration, thecommunications circuitry 218 and the programmer unit (not shown) use awire loop antenna and a radio frequency telemetric link, as is known inthe art, to receive and transmit signals and data between the programmerunit and communications circuitry 218. In this manner, programmingcommands and data are transferred between the ITCS device and theprogrammer unit during and after implant. Using a programmer, aphysician is able to set or modify various parameters used by the ITCSdevice. For example, a physician may set or modify parameters affectingsensing, detection, pacing, and defibrillation functions of the ITCSdevice, including pacing and cardioversion/defibrillation therapy modes.

Typically, the ITCS device is encased and hermetically sealed in ahousing suitable for implanting in a human body as is known in the art.Power to the ITCS device is supplied by an electrochemical power source220 housed within the ITCS device. In one configuration, the powersource 220 includes a rechargeable battery. According to thisconfiguration, charging circuitry is coupled to the power source 220 tofacilitate repeated non-invasive charging of the power source 220. Thecommunications circuitry 218, or separate receiver circuitry, isconfigured to receive RF energy transmitted by an external RF energytransmitter. The ITCS device may, in addition to a rechargeable powersource, include a non-rechargeable battery. It is understood that arechargeable power source need not be used, in which case a long-lifenon-rechargeable battery is employed.

FIG. 1D illustrates a configuration of detection circuitry 302 of anITCS device, which includes one or both of rate detection circuitry 310and morphological analysis circuitry 312. Detection and verification ofarrhythmias may be accomplished using rate-based discriminationalgorithms as known in the art implemented by the rate detectioncircuitry 310. Arrhythmic episodes may also be detected and verified bymorphology-based analysis of sensed cardiac signals as is known in theart. Tiered or parallel arrhythmia discrimination algorithms may also beimplemented using both rate-based and morphologic-based approaches.Further, a rate and pattern-based arrhythmia detection anddiscrimination approach may be employed to detect and/or verifyarrhythmic episodes, such as the approach disclosed in U.S. Pat. Nos.6,487,443; 6,259,947; 6,141,581; 5,855,593; and 5,545,186, which arehereby incorporated herein by reference in their respective entireties.

The detection circuitry 302, which is coupled to a microprocessor 306,may be configured to incorporate, or communicate with, specializedcircuitry for processing sensed cardiac signals in manners particularlyuseful in a transthoracic cardiac sensing and/or stimulation device. Asis shown by way of example in FIG. 1D, the detection circuitry 302 mayreceive information from multiple physiologic and non-physiologicsensors. As illustrated, transthoracic acoustics may be monitored usingan appropriate acoustic sensor. Heart sounds, for example, may bedetected and processed by non-electrophysiologic cardiac sensorprocessing circuitry 318 for a variety of purposes. The acoustics datais transmitted to the detection circuitry 302, via a hardwire orwireless link, and used to enhance cardiac signal detection. Forexample, acoustic information may be used in accordance with the presentinvention to quickly identify a cardiac signal within a group ofseparated signals, such as signals separated from a composite signalusing a blind source separation technique or a linear signal separationtechnique.

The detection circuitry 302 may also receive information from one ormore sensors that monitor skeletal muscle activity. In addition tocardiac activity signals, transthoracic electrodes readily detectskeletal muscle signals. Such skeletal muscle signals may be used todetermine the activity level of the patient. In the context of cardiacsignal detection, such skeletal muscle signals are considered artifactsof the cardiac activity signal, which may be viewed as noise. Processingcircuitry 316 receives signals from one or more skeletal muscle sensors,and transmits processed skeletal muscle signal data to the detectioncircuitry 302. This data may be used to discriminate normal cardiacsinus rhythm with skeletal muscle noise from cardiac arrhythmias.

As was previously discussed, the detection circuitry 302 is coupled to,or otherwise incorporates, noise-processing circuitry 314. The noiseprocessing circuitry 314 processes sensed cardiac signals to improve theSNR of sensed cardiac signals by reducing noise content of the sensedcardiac signals.

The components, functionality, and structural configurations depicted inFIGS. 1A-1D are intended to provide an understanding of various featuresand combination of features that may be incorporated in an ITCS device.It is understood that a wide variety of ITCS and other implantablecardiac monitoring and/or stimulation device configurations arecontemplated, ranging from relatively sophisticated to relatively simpledesigns. As such, particular ITCS or cardiac monitoring and/orstimulation device configurations may include particular features asdescribed herein, while other such device configurations may excludeparticular features described herein.

In accordance with embodiments of the invention, an ITCS device may beimplemented to include a subcutaneous electrode system that provides forone or both of cardiac sensing and arrhythmia therapy delivery.According to one approach, an ITCS device may be implemented as achronically implantable system that performs monitoring, diagnosticand/or therapeutic functions. The ITCS device may automatically detectand treat cardiac arrhythmias. In one configuration, the ITCS deviceincludes a pulse generator and one or more electrodes that are implantedsubcutaneously in the chest region of the body, such as in the anteriorthoracic region of the body. The ITCS device may be used to provideatrial and ventricular therapy for bradycardia and tachycardiaarrhythmias. Tachyarrhythmia therapy may include cardioversion,defibrillation and anti-tachycardia pacing (ATP), for example, to treatatrial or ventricular tachycardia or fibrillation. Bradycardia therapymay include temporary post-shock pacing for bradycardia or asystole.Methods and systems for implementing post-shock pacing for bradycardiaor asystole are described in commonly owned U.S. Patent Applicationentitled “Subcutaneous Cardiac Stimulator Employing Post-ShockTransthoracic Asystole Prevention Pacing, Ser. No. 10/377,274, filed onFeb. 28, 2003, which is incorporated herein by reference in itsentirety.

In one configuration, an ITCS device according to one approach mayutilize conventional pulse generator and subcutaneous electrode implanttechniques. The pulse generator device and electrodes may be chronicallyimplanted subcutaneously. Such an ITCS may be used to automaticallydetect and treat arrhythmias similarly to conventional implantablesystems. In another configuration, the ITCS device may include a unitarystructure (e.g., a single housing/unit). The electronic components andelectrode conductors/connectors are disposed within or on the unitaryITCS device housing/electrode support assembly.

The ITCS device contains the electronics and may be similar to aconventional implantable defibrillator. High voltage shock therapy maybe delivered between two or more electrodes, one of which may be thepulse generator housing (e.g., can), placed subcutaneously in thethoracic region of the body.

Additionally or alternatively, the ITCS device may also provide lowerenergy electrical stimulation for bradycardia therapy. The ITCS devicemay provide brady pacing similarly to a conventional pacemaker. The ITCSdevice may provide temporary post-shock pacing for bradycardia orasystole. Sensing and/or pacing may be accomplished using sense/paceelectrodes positioned on an electrode subsystem also incorporating shockelectrodes, or by separate electrodes implanted subcutaneously.

The ITCS device may detect a variety of physiological signals that maybe used in connection with various diagnostic, therapeutic or monitoringimplementations. For example, the ITCS device may include sensors orcircuitry for detecting respiratory system signals, cardiac systemsignals, and signals related to patient activity. In one embodiment, theITCS device senses intrathoracic impedance, from which variousrespiratory parameters may be derived, including, for example,respiratory tidal volume and minute ventilation. Sensors and associatedcircuitry may be incorporated in connection with an ITCS device fordetecting one or more body movement or body position related signals.For example, accelerometers and GPS devices may be employed to detectpatient activity, patient location, body orientation, or torso position.

The ITCS device may be used within the structure of an advanced patientmanagement (APM) system. Advanced patient management systems may allowphysicians to remotely and automatically monitor cardiac and respiratoryfunctions, as well as other patient conditions. In one example,implantable cardiac rhythm management systems, such as cardiacpacemakers, defibrillators, and resynchronization devices, may beequipped with various telecommunications and information technologiesthat enable real-time data collection, diagnosis, and treatment of thepatient. Various embodiments described herein may be used in connectionwith advanced patient management. Methods, structures, and/or techniquesdescribed herein, which may be adapted to provide for remotepatient/device monitoring, diagnosis, therapy, or other APM relatedmethodologies, may incorporate features of one or more of the followingreferences: U.S. Pat. Nos. 6,221,011; 6,270,457; 6,277,072; 6,280,380;6,312,378; 6,336,903; 6,358,203; 6,368,284; 6,398,728; and 6,440,066,which are hereby incorporated herein by reference.

An ITCS device according to one approach provides an easy to implanttherapeutic, diagnostic or monitoring system. The ITCS system may beimplanted without the need for intravenous or intrathoracic access,providing a simpler, less invasive implant procedure and minimizing leadand surgical complications. In addition, this system would haveadvantages for use in patients for whom transvenous lead systems causecomplications. Such complications include, but are not limited to,surgical complications, infection, insufficient vessel patency,complications associated with the presence of artificial valves, andlimitations in pediatric patients due to patient growth, among others.An ITCS system according to this approach is distinct from conventionalapproaches in that it may be configured to include a combination of twoor more electrode subsystems that are implanted subcutaneously in theanterior thorax.

In one configuration, as is illustrated in FIG. 2, electrode subsystemsof an ITCS system are arranged about a patient's heart 510. The ITCSsystem includes a first electrode subsystem, comprising a can electrode502, and a second electrode subsystem 504 that includes at least twoelectrodes or at least one multi-element electrode. The second electrodesubsystem 504 may include a number of electrodes used for sensing and/orelectrical stimulation.

In various configurations, the second electrode subsystem 504 mayinclude a combination of electrodes. The combination of electrodes ofthe second electrode subsystem 504 may include coil electrodes, tipelectrodes, ring electrodes, multi-element coils, spiral coils, spiralcoils mounted on non-conductive backing, screen patch electrodes, andother electrode configurations. A suitable non-conductive backingmaterial is silicone rubber, for example.

The can electrode 502 is positioned on the housing 501 that encloses theITCS device electronics. In one embodiment, the can electrode 502includes the entirety of the external surface of housing 501. In otherembodiments, various portions of the housing 501 may be electricallyisolated from the can electrode 502 or from tissue. For example, theactive area of the can electrode 502 may include all or a portion ofeither the anterior or posterior surface of the housing 501 to directcurrent flow in a manner advantageous for cardiac sensing and/orstimulation.

In accordance with one embodiment, the housing 501 may resemble that ofa conventional implantable ICD, is approximately 20-100 cc in volume,with a thickness of 0.4 to 2 cm and with a surface area on each face ofapproximately 30 to 100 cm². As previously discussed, portions of thehousing may be electrically isolated from tissue to optimally directcurrent flow. For example, portions of the housing 501 may be coveredwith a non-conductive, or otherwise electrically resistive, material todirect current flow. Suitable non-conductive material coatings includethose formed from silicone rubber, polyurethane, or parylene, forexample.

In addition, or alternatively, all or portions of the housing 501 may betreated to change the electrical conductivity characteristics thereoffor purposes of optimally directing current flow. Various knowntechniques may be employed to modify the surface conductivitycharacteristics of the housing 501, such as by increasing or decreasingsurface conductivity, to optimize current flow. Such techniques mayinclude those that mechanically or chemically alter the surface of thehousing 501 to achieve desired electrical conductivity characteristics.

As was discussed above, cardiac signals collected from subcutaneouslyimplanted electrodes may be corrupted by noise. In addition, certainnoise sources have frequency characteristics similar to those of thecardiac signal. Such noise may lead to over sensing and spurious shocks.Due to the possibility of relatively high amplitude of the noise signaland overlapping frequency content, filtering alone does not lead tocomplete suppression of the noise. In addition, filter performance isnot generally sufficiently robust against the entire class of noisesencountered. Further, known adaptive filtering approaches require areference signal that is often unknown for situations when a patientexperiences VF or high amplitude noise.

In accordance with one approach of the present invention, an ITCS devicemay be implemented to identify cardiac signals within a group ofseparated signals, such as those obtained from a blind source separation(BSS) technique. It is understood that all or certain aspects of thesignal identification technique described below may be implemented in adevice or system (implantable or non-implantable) other than an ITCSdevice, and that the description of the BSS technique as the methodseparation implemented in an ITCS device is provided for purposes ofillustration, and not of limitation. Devices and methods of blind sourceseparation are further described in commonly owned U.S. patentapplication Ser. No. 10/741,814, filed Dec. 19, 2003, herebyincorporated herein by reference. Devices and methods associated withanother useful signal separation approach that uses noise cancelingelectrodes are further described in commonly owned U.S. patentapplication Ser. No. 10/738,608, filed Dec. 17, 2003, herebyincorporated herein by reference.

Signal separation techniques provide for separation of many individualsignals from composite signals. For example, a composite signal detectedon or within a patient may contain several signal components producedfrom a variety of signal sources, such signal components includingcardiac signals, skeletal muscle movement related signals,electromagnetic interference signals, and signals of unknown origin.Signal separation techniques separate the composite signal intoindividual signals, but do not necessarily indicate the source of suchsignals.

As is disclosed in previously incorporated U.S. patent application Ser.No. 10/741,814, the use of the largest eigenvalues produced byperforming a principal component analysis on a composite signal matrixprovides one method of identifying the separated signals most likely tobe the cardiac signal of interest for ITCS devices. However, analyzingall the separated signals is a computationally intensive operation. Thepresent invention provides an indication of the signals most likely tobe the cardiac signal of interest in an efficient manner, therebygreatly decreasing the time necessary to identify the cardiac signal ofinterest from many possible separated signals.

Referring now to FIGS. 3 through 5, subcutaneous cardiac sensing and/orstimulation devices and methods employing cardiac signal separation aredescribed, which may be used to separate signals to discriminate and/oridentify a signal of interest within the separated signals. The mainprinciple of signal separation rests on the premise that spatiallydistributed electrodes collect components of a signal from a commonorigin (e.g., the heart) with the result that these components arestrongly correlated to each other in time. In addition, these componentsmay also be weakly correlated to components of another origin (e.g.,noise). An ITCS device may be implemented to separate these componentsaccording to their sources. To achieve this, the methods and algorithmsillustrated in FIGS. 3 through 5 may be implemented.

FIG. 3 illustrates a signal separation system 125 in accordance with thepresent invention. A signal separation process 414 is performed,providing a separated signal 419. The separated signal 419 is availablefor a variety of uses 420, such as arrhythmia detection, SVR(Supra-Ventricular Rhythm) confirmation, NSR (Normal Sinus Rhythm)confirmation, arrhythmia classification or other use.

FIG. 4 illustrates a signal separation methodology 150 in accordancewith the present invention. After initiating 402 the methodology, asignal record process begins 404. During the signal record event 404,one or more electrode signals are recorded for later processing, such asby signal separation processing. The recording may be continuous orperformed for a given period of time. At block 406, a determination ismade as to the presence or absence of an SVR using technology known inthe art. If an SVR is detected, no other action is necessary, andrecording and evaluation for SVR continues.

If SVR is absent, it is desirable to determine whether an adversecardiac condition exists, necessitating intervention, or whether thereis simply a spurious signal loss or other event not necessitatingintervention. A loss of SVR at decision 406 initiates a separationprocess 100. After the separation process 100 and any intervention stepsdeemed necessary are completed, a determination 408 is made as towhether other processing is necessary. If no further processing isnecessary, the recording process 404 continues. If further processing isnecessary, such additional processing 410 is performed, along with anyfurther action associated with processing 410, and then recording 404continues.

FIG. 5 illustrates another embodiment of a signal separation process100. A set of composite signals, including at least two and up to nsignals, are selected for separation, where n is an integer. Eachelectrode provides a composite signal associated with an unknown numberof sources. Pre-processing and/or pre-filtering 412 can be performed oneach of the composite signals. It may be advantageous to filter eachcomposite signal using the same filtering function. Signal separation414 is performed, providing at least one separated signal. The separatedsignal can then be used 420 for some specified purpose, such as toconfirm a normal sinus rhythm, determine a cardiac condition, define anoise signal, or other desired use.

If a treatment is desired, an appropriate treatment or therapy 418 isperformed. If continued signal separation is desired, the processreturns to perform such signal separation 414 and may iterativelyseparate 416 more signals until a desired signal is found, or allsignals are separated.

If there is no clear candidate cardiac signal, a process in accordancewith the present invention may be used to quickly search for the signalof interest (e.g., cardiac signal). This process may be repeated untilsuch a signal is found, or no more signals are separable as determinedby exceeding a predefined number of iterations, or some othertermination criterion.

FIG. 6 graphically depicts cardiac SNR improvement achievable by asignal separation methodology in accordance with the principles of thepresent invention. In this illustrative example, data was gathered underlow-SNR conditions with electrocautery noise using seven electrodesimplanted in the thoracic region of a pig. The bottom subplot,identified as trace 452, represents the raw implanted electrode signal.The next subplot, identified as trace 450, shows this signal after inputfiltering for optimal raw SNR using a linear-phase (4^(th)-order Bessel)band pass filter from 5 to 20 Hz. The top two subplots, identified astrace 448 and trace 446, illustrate the resulting separated componentsassociated with the two largest eigenvalues. In this example, trace 446is associated with the electrocautery signal, having the largesteigenvalue. Trace 448 is the uncorrupted cardiac signal.

An ITCS device may, for example, employ a hierarchical decision-makingprocedure that initiates a blind source separation algorithm when noiseor possible arrhythmia is detected. By way of example, a local peakdensity algorithm or a curvature-based significant point methodology maybe used as a high-level detection routine.

The cardiac signal may be identified among the few (e.g., two or three)largest separated signals, however examining the entirety of allcandidate signals is computationally intensive. Information from anon-electrophysiologic sensor, such as those described earlier, may beused to focus the search for the cardiac signal among the candidatesignals. A signal independent of cardiac electrical activity, such as anacoustic signal of cardiac heart-sounds, an accelerometer, a bloodsensor, or other non-electrophysiologic source sensor, may be used toimprove the detection and classification of the cardiac signal from theseparated signals.

In an embodiment of the present invention, heart sounds are used to aidin signal discrimination when detecting various heart rhythms in thepresence of electrical noise and/or artifacts. Because the additionaldiscriminating non-electrophysiologic signal is time correlated withrespect to the cardiac electrophysiological signals, thenon-electrophysiologic signal may provide information about a patient'srhythm state even in the presence of electrical noise.

In one embodiment, a subcutaneous sensor, such as an accelerometer oracoustic transducer, may be used to detect heart sounds. The heartsounds may be used together with rate, curvature, and other ECGinformation to discriminate normal sinus with electrical noise frompotentially lethal arrhythmias such as ventricular tachycardia andventricular fibrillation. An ITCS device may utilize one or more of thepresence, characteristics, and frequency of occurrence of the heartsound combined with ECG information when performing signal or rhythmdiscrimination.

A heart rate determined from the ECG signal may, for example, beanalyzed along with heart sound information for diagnostic purposes.High ECG heart rate detection along with normal rate heart sounds wouldindicate the presence of noise in the ECG signal. High ECG heart ratedetection along with modified heart sounds would indicate a potentiallylethal arrhythmia. It is noted that ECG morphology or other techniquescould replace rate in the example above. It should also be noted thatother sensor derived signals could replace heart sounds. For example,impedance, pulse pressure, blood volume/flow, or cardiac accelerationscould be used.

Various types of acoustic sensors may be used to detect heart sounds.Examples of such acoustic sensors include diaphragm based acousticsensors, MEMS-based acoustic sensors such as a MEMS-based acoustictransducer, fiber optic acoustic sensors, piezoelectric sensors, andaccelerometer based acoustic sensors and arrays. These sensors may beused to detect the audio frequency pressure waves associated with theheart sounds, and may also be used to detect othernon-electrophysiologic cardiac related signals.

The presence of cardiac pulse, or heartbeat, in a patient is generallydetected by palpating the patient's neck and sensing changes in thevolume of the patient's carotid artery due to blood pumped from thepatient's heart. A graph of a carotid pulse signal 810, representativeof the physical expansion and contraction of a patient's carotid arteryduring two consecutive pulses, or heartbeats, is shown at the top ofFIG. 7. When the heart's ventricles contract during a heartbeat, apressure wave is sent throughout the patient's peripheral circulationsystem. The carotid pulse signal 810 shown in FIG. 7 rises with theventricular ejection of blood at systole and peaks when the pressurewave from the heart reaches a maximum. The carotid pulse signal 810falls off again as the pressure subsides toward the end of each pulse.

The opening and closing of the patient's heart valves during a heartbeatcauses high-frequency vibrations in the adjacent heart wall and bloodvessels. These vibrations can be heard in the patient's body as heartsounds, and may be detected by sensors, as described earlier. Aconventional phonocardiogram (PCG) transducer placed on a patientconverts the acoustical energy of the heart sounds to electrical energy,resulting in a PCG waveform 820 that may be recorded and displayed, asshown by the graph in the upper middle portion of FIG. 7.

As indicated by the PCG waveform 820 shown in FIG. 7, a typicalheartbeat produces two main heart sounds. A first heart sound 830,denoted S1, is generated by vibration generally associated with theclosure of the tricuspid and mitral valves at the beginning of systole.Typically, the heart sound 830 is about 14 milliseconds long andcontains frequencies up to approximately 500 Hz. A second heart sound840, denoted S2, is generally associated with vibrations resulting fromthe closure of the aortic and pulmonary valves at the end of systole.While the duration of the second heart sound 840 is typically shorterthan the first heart sound 830, the spectral bandwidth of the secondheart sound 840 is typically larger than that of the first heart sound830.

An electrocardiogram (ECG) waveform 850 describes the electricalactivity of a patient's heart. The graph in the lower middle portion ofFIG. 7 illustrates an example of the ECG waveform 850 for two heartbeatsand corresponds in time with the carotid pulse signal 810 and PCGwaveform 820 also shown in FIG. 7. Referring to the first shownheartbeat, the portion of the ECG waveform 850 representingdepolarization of the atrial muscle fibers is referred to as the “P”wave. Depolarization of the ventricular muscle fibers is collectivelyrepresented by the “Q.” “R,” and “S” waves of the ECG waveform, referredto as the QRS complex. Finally, the portion of the waveform representingrepolarization of the ventricular muscle fibers is known as the “T”wave. Between heartbeats, the ECG waveform 850 returns to anisopotential level.

Fluctuations in a patient's transthoracic impedance signal 860 alsocorrelate with blood flow that occurs with each cardiac pulse wave. Thebottom graph of FIG. 7 illustrates an example of a filteredtransthoracic impedance signal 860 for a patient in which fluctuationsin impedance correspond in time with the carotid pulse signal 810, thePCG waveform 820, and ECG waveform 850, also shown in FIG. 7.

Referring now to FIG. 8, in another embodiment of the present inventioninvolving heart sounds, such sounds may be used for selection ofseparated signals. In general terms, this approach provides forselection of the ECG signal from a group of separated signals. A blindsource separation (BSS) technique may be employed, it being understoodthat other signal separation techniques that provide for a separatedcardiac signal from a set of detected signals may be used (e.g., thetechnique described in previously incorporated U.S. patent applicationSer. No. 10/738,608). This technique can separate cardiac signals fromnoise and artifacts in a cardiac monitoring system (either external orimplantable), such as an ITCS device for example. The BSS technique isused to separate signals, and, in a primitive form, provides noinformation as to which signal is the cardiac signal. This informationmay be derived from other algorithms following the BSS signal separationoperation, such as by use of a heart sound signal or othernon-electrophysiologic signal.

FIG. 8 is a graph illustrating two consecutive PQRS complexes in the ECGsignal 850 and their associated non-electrophysiological componentsdeveloped from an accelerometer signal 835. Also illustrated is adetection window 870 that is used to evaluate correlation of the signalsin accordance with an embodiment of the present invention. As isillustrated in FIG. 8, an S1 heart sound 832 and an S1 heart sound 834are, in general, closely time correlated with a QRS complex 852 and aQRS complex 854, respectively. The S1 heart sound 832, an S2 heart sound833, and the S1 heart sound 834 are illustrated as detected from aninternally implanted accelerometer. The S1 heart sound may provide aclose time correlation with cardiac signals but not with noise andartifact signals. As such, heart sounds may be used to discriminate aseparated cardiac signal from other separated signals.

An ITCS device may be implemented to include signal processing circuitryand/or signal processing software as illustrated in FIGS. 1C and 1D.With continued reference to FIG. 8, signal processing may be used tocorrelate heart sounds, such as the S1 heart sound, with R-wave peaks orother QRS complex features to allow selection of the correct signal(i.e., the cardiac signal) after blind source separation or otherseparation technique has separated the ECG signal from various othersignals.

In the approach illustrated in FIG. 8, the ITCS algorithm firstidentifies the S1 heart sound 832. An examination or detection window870 is then defined to start at least before the peak of the S1 heartsound 832, illustrated as a preceding window start time 875. Thealgorithm then looks for time correlation between peak amplitudes on oneof the separated channels with this examination window 870. The signaldemonstrating highest correlation is designated to be the ECG signal.For example, the ECG signal 850 has an R-wave peak 856 falling withinthe examination window 870, and an R-wave peak 858 falling within anexamination window 872. The R-wave peak 856 falling within theexamination window 870 produces a large correlation value, indicatingthat the ECG signal 850 is time correlated to the S1 heart sound signal832 within the examination window 870. Similarly, the R-wave peak 858falling within the examination window 872 produces a large correlationvalue, indicating that the ECG signal 850 is time correlated to the S1heart sound signal 834 within the examination window 872.

This approach to cardiac signal discrimination involves use of anon-electrophysiologic signal (e.g., the accelerometer signal 835) toselect the ECG signal 850 from the various separated signals. Thealgorithm is robust, in that the heart sound information is notelectrophysiological in nature, and therefore not susceptible to thesame noise sources as the ECG signal 850.

FIG. 9 illustrates the signal separation process 100 shown in FIG. 4,including various processes associated with a signal separationtechnique involving the use of heart sounds in accordance with thepresent invention. As shown in FIG. 9, signals are collected 700 fromspatially diverse electrodes and one or more heart sound sensors (e.g.,acoustic sensor(s)). A BSS routine, such as that discussed above, isperformed 702 producing a number of separated signals 704. A counter 706is set to a desired maximum number, N, of separated signals 704, and theseparated signal counter 706 is initialized.

A heart sound/QRS complex correlation operation 708 is performed. If adetected heart sound is correlated with the QRS complex, then the sensedsignal is deemed a cardiac signal 710. If the detected heart sound isnot correlated with the QRS complex, the signal counter is compared 714with the counter threshold, N. If the signal counter 706 is not greaterthan the counter maximum number, N, then the signal counter 706 isincremented at step 712 and the correlation operation 708 is repeated.If the signal counter 706 is greater than the counter maximum number, N,then the sensed signal is deemed indeterminate 716, and otherhierarchical processes may be initiated.

An ITCS device may operate in a batch mode or adaptively, allowing foron-line or off-line implementation. To save power, the system mayinclude the option for a hierarchical decision-making routine that usesalgorithms known in the art for identifying presence of arrhythmias ornoise in the collected signal and judiciously turning on the cardiacsignal extraction routine of the present invention.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

What is claimed is:
 1. A signal separation method, comprising: detectinga composite electrical signal at a subcutaneous non-intrathoraciclocation, the composite electrical signal associated with a plurality ofsources; receiving information associated with anon-electrophysiological cardiac source; separating a signal from thecomposite electrical signal using source separation; and verifying thatthe separated signal is a cardiac signal using the separated signal andthe non-electrophysiological cardiac source information.
 2. The methodof claim 1, wherein verifying that the separated signal is the cardiacsignal comprises providing a detection window defined by a start timeand a stop time determined using the non-electrophysiological cardiacsource information.
 3. The method of claim 2, further comprisingdetecting a QRS complex within the detection window.
 4. The method ofclaim 1, wherein the non-electrophysiological cardiac source informationcomprises acoustic emission information.
 5. The method of claim 1,wherein the non-electrophysiological cardiac source informationcomprises a temporal location of a peak heart-sound.
 6. The method ofclaim 5, wherein verifying that the separated signal is the cardiacsignal comprises providing a detection window defined by a start timepreceding the temporal location of a peak heart-sound.
 7. The method ofclaim 1, wherein the non-electrophysiological cardiac source informationcomprises blood-flow information.
 8. The method of claim 1, wherein thenon-electrophysiological cardiac source information comprises pulsepressure information.
 9. The method of claim 1, wherein thenon-electrophysiological cardiac source information comprises pulseoximetry information.
 10. The method of claim 1, wherein thenon-electrophysiological cardiac source information comprisestransthoracic impedance information.
 11. The method of claim 1, whereinverifying that the separated signal is the cardiac signal comprisesproviding a detection window within which the cardiac signal iscorrelated to a signal associated with the non-electrophysiologicalcardiac source.
 12. The method of claim 1, further comprisingdetermining a time separation between a peak of the separated signal anda peak of a signal associated with the non- electrophysiological cardiacsource.
 13. The method of claim 12, wherein the time separation is usedto identify a cardiac signal.
 14. The method of claim 1, wherein thesignal is separated from the composite electrical signal using blindsource separation.
 15. The method of claim 14, wherein the blind sourceseparation comprises an independent component analysis performed on thecomposite electrical signal.
 16. The method of claim 1, furthercomprising detecting a cardiac condition using the separated signal. 17.The method of claim 1, further comprising detecting a cardiac conditionusing the separated signal by performing a correlation between theseparated signal and a signal associated with thenon-electrophysiological cardiac source.
 18. The method of claim 1,further comprising detecting a cardiac arrhythmia using the cardiacsignal.
 19. The method of claim 18, further comprising treating thecardiac arrhythmia.
 20. An implantable device, comprising: means forsubcutaneously detecting a composite electrical signal associated with aplurality of signal sources; means for subcutaneously detectingnon-electrical cardiac activity; means for separating a signal from thecomposite electrical signal using source separation; and means fordetermining whether or not the separated signal is a cardiac electricalsignal using the detected non-electrical cardiac activity.
 21. Thedevice of claim 20, wherein the determining means comprises means forperforming a time correlation between the separated signal and a signalassociated with the detected non-electrical cardiac activity.
 22. Thedevice of claim 20, wherein the determining means comprises means forevaluating the separated signal within a detection window.
 23. Thedevice of claim 22, further comprising means for determining a starttime to initiate the detection window.
 24. The device of claim 20,further comprising means for detecting an arrhythmia using the cardiacelectrical signal.
 25. The device of claim 24, further comprising meansfor treating the arrhythmia.
 26. The device of claim 20, furthercomprising means for discriminating cardiac rhythms.